Outwardly deploying airbag systems have been developed as pedestrian-protecting airbag systems that prevent a pedestrian or the like from directly hitting, for example, a wind shield by deploying an airbag along an outer surface of a vehicle body in order to protect the pedestrian or the like when a moving vehicle collides with, for example, the pedestrian, a bicycle, or a motorcycle.
When the internal pressure of an outwardly deploying airbag inflated along the outer surface of the vehicle body is excessively high or excessively low, an impact on the pedestrian or the like cannot be reduced sufficiently.
Japanese Unexamined Patent Application Publication No. 8-276816 describes an outwardly deploying airbag system in which the internal pressure of an airbag increases as the speed of a car increases. This publication also discloses a mechanism that increases the inflation internal pressure of the airbag by reducing the internal capacity of the airbag, and discloses that the internal pressure of the airbag is decreased by inhibiting a part of gas from an inflator from being filled in the airbag.
It is known that vent holes are provided in an airbag of an outwardly deploying airbag system, as is described in, for example, Japanese Unexamined Patent Application Publication No. 2001-315599.
The airbag described in Japanese Unexamined Patent Application Publication No. 8-276816 does not have a vent hole, and the internal pressure of the inflated airbag is kept high for a relatively long period. In this type of airbag, gas does not leak from the vent hole when a pedestrian or the like hits the airbag, and therefore, a sufficient impact absorption property is not necessarily ensured.
When the airbag is provided with a vent hole, gas leaks from the airbag through the vent hole. Therefore, it is preferable to increase the output of the inflator in expectation of gas leakage. In this case, however, the maximum internal pressure of the inflated airbag vainly increases. Consequently, an impact on the pedestrian or the like cannot be sufficiently reduced, and, for example, the sewing cost and base-cloth cost of the airbag are increased.